Sudden onset of wind

I have a 4 month old who has been a very laid back baby. Apart from a few issues with weight gain and feeding everything has been lovely! Because of his low weight it was suggested I put him onto formula. I've compromised and he has a bottle before bed and growing well (last weigh in was 3 weeks ago).

Anyway over the last 2 weeks or so all of a sudden he needs winding. he has never needed it before but gets so uncomfortable laying down. I've put him on infacol and gripe water which does help him to burp but I'm wondering if it's anything I've done to cause it? He's also now turning into a very sicky baby after feeds...again a complete change from the no sick no burp baby I had before. any suggestions?